Novel Gd3+-doped silica-based optical fiber material for dosimetry in proton therapy

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Optical fibers hold promise for accurate dosimetry in small field proton therapy due to their superior spatial resolution and the lack of significant Cerenkov contamination in proton beams. One known drawback for most scintillation detectors is signal quenching in areas of high linear energy transfer, as is the case in the Bragg peak region of a proton beam. In this study, we investigated the potential of innovative optical fiber bulk materials using the sol-gel technique for dosimetry in proton therapy. This type of glass is made of amorphous silica (SiO{}_{2}) and is doped with Gd{}^{3+} ions and possesses very interesting light emission properties with a luminescence band around 314 nm when exposed to protons. The fibers were manufactured at the University of Lille and tested at the TRIUMF Proton Therapy facility with 8.2–62.9 MeV protons and 2–6 nA of extracted beam current. Dose-rate dependence and quenching were measured and compared to other silica-based fibers also made by sol-gel techniques and doped with Ce{}^{3+} and Cu{}^{+}. The three fibers present strong luminescence in the UV (Gd) or visible (Cu,Ce) under irradiation, with the emission intensities related directly to the proton flux. In addition, the 0.5 mm diameter Gd{}^{3+}-doped fiber shows superior resolution of the Bragg peak, indicating significantly reduced quenching in comparison to the Ce{}^{3+} and Cu{}^{+} fibers with a Birks’ constant, k{}_{B}, of (0.0162 pm 0.0003) cm/MeV in comparison to (0.0333 pm 0.0006) cm/MeV and (0.0352 pm 0.0003) cm/MeV, respectively. To our knowledge, this is the first report of such an interesting k{}_{B} for a silica-based optical fiber material, showing clearly that this fiber presents lower quenching than common plastic scintillators. This result demonstrates the high potential of this inorganic fiber material for proton therapy dosimetry.